THE GENERAL ANIMAL FUNCTIONS ' 75 



volving only one parent is asexual. It usually occurs when the 

 adult size of the animal is attained. It is not confined to the 

 Protozoa or single-celled animals, but may occur in several In- 

 vertebrate groups in which (Hydra, Fig. 8i) there is not a high 

 degree of specialization. The budded individual or offspring 

 may consist of one cell or of many. In addition to the internal 

 stimulus afforded by the attainment of normal size, external 

 conditions such as diminished food supply, temperature changes, 

 etc., influence the process of non-sexual reproduction. 



loi. Sexual Reproduction. — There are some evidences that 

 even in the one-celled animals the method of reproduction by 

 division cannot be continued indefinitely without some ill effects 

 to the organism. After a long series of divisions, a period of 

 rest or a change of living conditions, or some other stimulus 

 seems to be necessary to restore reproduction. In many Pro- 

 tozoa there is at certain times a union of two individuals, either 

 temporarily or permanently, accompanied by exchange of 

 nuclear material or by a fusion of the whole protoplasm. After 

 a period of rest and the coming of favorable conditions of life, 

 this union seems to renew activity of division. Something sim- 

 ilar is seen in the more complex animals — the Metazoa. After 

 a period of cell divisions, by which the individual body is built 

 up, the majority of cells, as muscle or nerve cells, appear to 

 lose their power of dividing, and even the less differentiated 

 cells which we have described as the ova and sperm, which 

 arise from the primordial germ cells (§49), are ordinarily in- 

 capable of continuing the division necessary to produce a new 

 individual until they have been stimulated by union with each 

 other (or by some artificial means). Such unions of cells, to 

 form by later divisions a new individual, are called conjugation 

 or fertilization, and the new individual which results is said to 

 arise by sexual reproduction. The uniting cells may be similar 

 (as in Pandorind), in which case the union is isogamous. More 

 usually the cells are different, and the union is heterogamous. 

 In the latter case the cells are called ovum and sperm (Chapter 

 IV) and are usually formed in different individuals, though 

 both classes of cells may arise (hermaphroditism) from different 



